1. Field of the Invention
This invention relates to polyarylene sulfides and a process for preparation thereof. In particular, the invention relates to polyarylene sulfides exhibiting excellent mechanical properties, heat resistance and chemical resistance properties and which have a narrow molecular weight distribution and also to a process for preparing polyarylene sulfides of such advantageous characteristics. More particularly, the invention concerns a process for preparing such polyarylene sulfides by ring opening polymerization of cyclic arylene sulfide oligomers.
2. Prior Art
Polyarylene sulfides have excellent heat and chemical resistance properties and, hence, have attracted a good deal of attention as materials useful for producing parts for electrical and electronic instruments, automotive parts and the like. Polyarylene sulfide may be formed or molded, for example by injection or extrusion molding techniques, into various parts, films, sheets, fibers and the like. These molded products have been used in a wide range of applications where high heat and chemical resistance properties are required.
Generally, the known processes for preparing polyarylene sulfides are based on a desalting condensation polymerization involving a nucleophilic reaction between a dihalo-aromatic compound and an alkali metal compound, such as sodium sulfide, in an organic amide solvent, such as N-methyl pyrrolidone, see for example Japanese Patent Publication, KOKOKU, No. 45-3368. However, since this type of processes involve the low reactive aromatic nucleophilic reaction, it is necessary to effect the processes at a high temperature and a high pressure. Further it is required to employ a high boiling polar solvent, such as N-methyl pyrrolidone, that is very expensive. The consequent recovery of the expensive solvent consumes a large amount of energy and accordingly adds to the process cost. Furthermore, the processes produce an undesirably large proportion of by-product salt, such as sodium chloride, which requires additional steps for removing the by-product salt, such as washing and separation steps with making the process and apparatus complicated. It is difficult to completely remove the by-product salt by the most conventional washing, for example, with water.
In addition, the polymers produced by the processes contain chlorine and sodium atoms at the ends of the polymeric chains. Commercially available polyarylene sulfides have usually a sodium content on the order of 1,000-3,000 p.p.m. and a chlorine content on the order of 2,000-4,000 p.p.m. The alkali metal salt residue at such a level in the product polymers will present difficulties, for example deterioration of the physical properties, such as electrical property. For example, in the case where a polyarylene sulfide material having high contents of sodium and chloride is used for sealing or encapsulating electronic devices on parts, such as IC, the insulation of circuit would be deteriorated by moisture uptake or the members, such as electrode and lead wire frame, would be carroded or even disconnected. These would result in deterioration of the characteristics of electronic elements or damage and fault of the elements. These undesirable phenomena have been recognized in the art.
It has been noted that the polyarylene sulfides produced by the known processes have, generally, a very wide range of molecular weight distribution, typically from 5 to 20 as expressed as ratio of the weight average molecular weight, Mw, to the number average molecular weight, Mn (i.e. Mw/Mn ratio). the higher molecular weight components or fractions in a polyarylene sulfide product tend to contribute to poor flowability and formability, while the lower molecular weight components or fractions contributing to poor mechanical and chemical properties and others.
Recently, a plenty of attention has been focussed on development of novel processes for producing polyarylene sulfides. A typical example is a process in which diphenyl disulfide or thiophenol is subjected to cationic-oxidative polymerization with a Lewis acid (see Japanese Patent Public Disclosures. KOKAIs, Nos. 63-213526 and 63-213527). Another example is a process in which diphenyl disulfide or thiophenol is subjected to oxidative coupling polymerization with oxygen using a catalyst in the presence of an acid (see Japanese Patent Public Disclosure, KOKAI, No. 2-169626). According to the recently proposed processes, unbranched polyphenylene sulfides may be produced at a high yield under mild conditions. However, generally the products exhibit such a low melting point and molecular weight that they as produced cannot be employed successfully in molding applications, for example in injection molding. Furthermore, the processes employ a large amount of expensive Lewis acid or oxidant and require a prolonged reaction time. These difficulties give rise to an increased production cost and other disadavantages in the industrial practice.
Methods for reducing the content of the residual by-product salt in polyarylene sulfides have been also proposed, for example, a method in which the crude polymer is washed with hot water (see Japanese Patent Public Disclosure, KOKAI, No. 55-156342), and a method in which the crude polymer is treated in an aromatic solvent at an elevated temperature (see Japanese Patent public Disclosure, KOKAI, No. 59-219331). Although these methods are effective for removal of the residual sodium content, they are not satisfactory for removal of the residual chlorine content and the polymer treated thereby may still contain the residual chlorine on the order of 2,000-3,000 p.p.m.
Japanese Patent Public Disclosure, KOKAI, No. 62-106929 proposes a method for reducing the residual chlorine content in polyarylene sulfide in which the crude polymer is treated with an alkali metal salt of mercapto group-containing compound at an elevated temperature. A highly pure polymer can be obtained by the method, but the product polymer has an unduly broad molecular weight distribution corresponding to Mw/Mn ratio of about 6-20.
On the other hand, a method for obtaining a polyarylene sulfide of a narrow molecular weight distribution is disclosed in Japanese Patent public Disclosure, KOKAI, No. 2-182727, which method comprising washing the crude polymer with an organic polar solvent at an elevated temperature. However, this method is not effective for removal of the chlorine content, so that the product polymer may have a residual chlorine content on the order of 2,000-3,000 p.p.m.
As above-discussed, techniques which are effective both for narrowing the molecular weight distribution of polyarylene sulfides and for reducing the contents of residual alkali metal and halogen in polyarylene sulfides have not been established.
Now, poly(phenylene sulfide) oligomers are known to be utilizable for preparing useful cured products, see U.S. Pat. No. 4,046,749. This patent discloses a method in which poly(phenylene sulfide) oligomers are heated at elevated temperatures in the presence of oxygen gas. The products are partially cross-linked via oxygen linkages. The resulting cured products not only do not show definite melting points, but exhibit unsatisfactory flowability properties and give raise to poor or unacceptable coloring in the molded products.
Generally, it is known that cyclic oligomers, for example .epsilon.-caprolactam, may be polymerized by the ring opening polymerization technique so as to form polymers, including useful engineering plastics. For example, as well known, .epsilon.-caprolactam may be "ring-opening" polymerized to give nylon-6.
Recently, various reports have been made with respect to processes for producing large cyclic oligomers, methods for polymerizing such oligomers, and applications of the resulting polymers in production of composite materials. See, for example:
polycarbonate: ACS Polym. Prepr. 30 [2], 569 (1989) PA1 polyarylate: ACS Polym. Prepr. 30 [2], 579 (1989) PA1 polyether sulfone: ACS Polym. Prepr. 30 [2], 581 (1989) PA1 polyimido siloxane: Macromolecules, 23, 4341 (1990); ibid. 23, 4514 (1990) PA1 aromatic polyether: J. Chem. Soc, Chem. Commun. 1990, 336 PA1 Japanese Patent Public Disclosure, KOKAI. No. 3-88828 PA1 providing at least one cyclic arylene sulfide aligomer of the formula ##STR2## wherein S represents a sulfur atom, groups Ar, which may be the same or different, represent each an arylene radical having 6-24 carbon atoms, groups R, which may be the same or different, represent each a C.sub.1 -C.sub.12 alkyl radical, or a C.sub.1 -C.sub.12 alkoxy radical, a C.sub.6 - C.sub.24 arylene radical, or a primary, secondary, or tertiary amino group, n is an integer of from 2 to 50, and m is an integer of from 0 to 15, and PA1 heating said cyclic oligomer I in the presence of a ring opening polymerization catalyst so as to form polyarylene sulfide.